1. Field of the Invention
This invention relates to a process for manufacturing chlorine dioxide by the reaction of a chlorate and hydrochloric acid in the presence of a catalyst.
2. Description of the Prior Art.
Chlorine dioxide is a commercially important material in such fields as pulp bleaching and fat decoloring, and also has recently been used for the purpose of environmental protection and pollution abatement as in the removal of phenols from industrial sewage and in the denitration of industrial waste gases. Thus, it is highly desirable to have a process by which chlorine dioxide can be manufactured economically and safely.
A processs has been known in which sodium chlorate is reduced with sodium chloride in the presence of sulfuric acid at an elevated concentration. However, according to such process, a large amount of sulfuric acid is contained in the process effluents, rendering the process highly uneconomical unless recovery is made of such acid.
Furthermore, since sulfur is now commercially available at a reduced cost owing to the recently increasing introduction of desulfurization facilities, the sulfur compounds found in such effluents like sulfuric acid and sodium sulfate are losing their commercial value.
A process for reducing the amount of such byproduct sulfur compounds is described in Japanese Pat. No. 15890/1968 by which sodium sulfate is removed in the form of solid anhydrous sodium sulfate by maintaining the acidity of the reaction medium sufficiently low. Even with this process, however, 2.2 kg of sodium sulfate is produced for 1 kg of chlorine dioxide.
For the purpose of further reducing the amount of the byproduct sulfur compounds, another process was proposed to convert the byproduct sodium sulfate to sodium chloride using anhydrous hydrogen chloride (Pulp and Paper Canada Vol. 75 (10) 53-56 (1974)). This process is disadvantageous, however, in that it requires the use of expensive anhydrous hydrogen chloride and the installing of a second reactor, in addition to the chlorine dioxide generator, in which sodium sulfate and anhydrous hydrogen chloride are to be reacted.
One of the processes utilizing low acidities like the one mentioned above is described in U.S. Pat. No. 3,563,702 in which a catalyst selected from the group consisting of silver ions, manganese ions, vanadium pentoxide, dichromate ions and arsenic ions is used. Although it is claimed that all of these catalysts are equally effective, the inventors of this invention have found that the catalytic activity of silver ions, dichromate ions, and manganese ions is, respectively, 1000:10:1, vanadium pentoxide and arsenic ions being far less effective than manganese ions. In conclusion, when used alone as the catalyst, only the silver ions are useful commercially.
Still another process for reducing the amount of the byproduct sodium sulfate comprises feeding an alkali metal chlorate and hydrochloric acid into a single generator-crystallizer, thereby depositing the corresponding alkali metal chloride instead of the undesirable byproduct sulfur compounds. Such a process, as is described in Japanese Pat. No. 15391/1972, when utilizing sodium chlorate as the alkali metal chlorate, is expressed by the following equations: EQU NaClO.sub.3 +2HCl.fwdarw.ClO.sub.2 +1/2Cl.sub.2 +NaCl+H.sub.2 O (1) EQU NaClO.sub.3 +6HCl.fwdarw.3Cl.sub.2 +NaCl+3H.sub.2 O (2)
The reaction medium has an acidity of 0.05 to 1.9 normal and is practically saturated with sodium chloride. A catalyst selected from the group consisting of vanadium pentoxide, silver ions, manganese ions, dichromate ions, and arsenic ions is used in this process. At such high chloride concentrations, however, the silver ions, being the only catalyst that is useful commercially for processes utilizing sulfuric acid as a strong acid, coprecipitate with the solid alkali metal chloride that is deposited and are removed from the reaction medium. As a result, the silver ion concentration of the reaction medium is reduced considerably, increasing the portion of reaction (2) and lowering the efficiency of chlorine dioxide generation. Thus, the silver ions can not be regarded as a useful catalyst for the hydrochloric process.